Auxiliary air heater



AUXILIARY AIR HEATER Filed Sept. 14, 1942 o o 0000 o o @504 B 55633" INVENTORS ogggggggggnpgg 60011 4 If Jamison E 21rd J. Talia r5021 Patented Mar. 5, 1946 AUXILIARY AIR HEATER Flushing, and Ward S.

George P. Jackson,

Patterson, Chappaqua, N. Y., bastion Engineering Company, Inc.,

assignors to Com- New York,

Application September I4, 1942, Serial No. 458,230 (Cl. 110-56) 2 Claims.

This invention relates to heat exchange apparatus and particularly to air heaters combined with conventional steam boilers.

In pulverizing mills the fuel is dried durin grinding by heated air which vaporizes and carries oil the moisture from the fuel. Low grade fuels may have a total moisture content of nearly 20% most of which is-suriace moisture that adversely affects the pulverizing capacity of a mill unless suiiicient heat is supplied by the air to vaporize the moisture during the grinding process. Some mills dry fuels better with a limited amount of high temperature air than with a larger amount of lower temperature air. The air temperature required in the pulverizer may be as high as or higher than the temperature of flue gas leaving the boiler whose furnace is flred from the mill. With the conventional air heater located beyond the generating surfaces of the boiler with respect to gas flow it is sometimes impossible to obtain the desired air temperature for drying the fuels within a mill because the gases have been cooled too much by the steam generating and superheating surfaces of the boiler.

An object of this invention is to provide in a steam generating unit associated with an air heater, an improved auxiliary air heater for delivering air at a temperature equal to or higher than the flue gas temperature leaving the boiler.

In theaccompanying drawing:

Figure 1 is a schematic elevational view of a steam generating unit embodying the invention;

Figure 2 is a horizontal cross section on line 2-2 of Fig. 1. v

The steam generating unit comprises an upper steam and water drum l6 and a lower drum l2 interconnected by steam generating tubes [2 which, as shown, are grouped in banks i4, i and I6. Between banks i4 and I5 is a steam superheater [1. Baiiies 26, 2|, 22 and 23 are arranged to cause the gases from the furnace 24 to move upwardly over superheater l1, downwardly over bank i5 and upwardly over bank l6.

The boiler furnace 24 receives its fuel from burners I i to which the fuel is delivered in flotation in an air stream via conduit l3 from fan i5. Fan l5 draws air through a mill 21 within which the fuel is pulverized and removed by the carrier air. Raw coal enters the mill through hopper 23 and while being ground has its moisture vaporized by hot air passing therethrough.

At the rear of the boiler is a tubular air heater 25 comprising a bank of tubes 26 surrounded by casing 21 having baflles 26 extending at spaced intervals across tube bank 26. A duct 36 connects the top ends of the air tubes 26 with the upper end of the gas pass in which tube bank It is located for directing the flue gases leaving the boiler into the tubes of the air heater 25. A duct 3| connects the bottom ends of the tubes 26 to the chimney. The casing 21 of the air heater is connected at its lower end to a conduit 32 from which it receives fresh air under pressure and at its upper end is connected to a conduit 33 into which it delivers heated air finally discharged to a chamber 34 at the front of the furnace which surrounds the burners l'l. Air in passing upwardly through the casing 21 of air heater 25 is caused by the baiiies tubes 26. Inasmuch as the air heater 25 utilizes flue gases at the temperature at which they leave the boiler for its heating. medium, the heated air in conduit 33 will necessarily be at a temperature lower than that of the gases leaving the boiler. To obtain a higher air temperature, the boiler could be redesigned so as to have less heat absorbing surface and thereby deliver to the air heater flue gases at a higher temperature. This expedient is not usually desirable-where a standardboiler design is usually followed in manufacturing because it necessitates changes from the usual design which involve added costs.

-According tothe invention the standard design oi. boiler of a given type is maintained and an auxiliary air heater 40 is provided which comprises rows of elements or tubes 4| preferably occupying space within a bank of boiler tubes, such as bank i6. The space for tubes 4| is created by omission of some of the boiler tubesv i2.in bank l6. Obviously'the air heater tubes 4i may be placed within any bank and may also be placed between adjacent tubes of a bank instead of replacing some of them. The top ends of the auxiliary air heater tubes 4| are connected to conduit 33 thereby receiving the heated air from air heater 25. Because the gases within a tube bank of the boiler are hotter than when they finally leave the boiler, the air passing through the auxiliary air heater will be superheated to a temperature above that of the air leaving air heater 25 via conduit 33. Preferably air travels counterflow to the direction of gas travel.

The bottom of the tubes 4! of the auxiliary air heater are connected to a conduit 42 which directs the superheated air to the coal pulverizing mill 2!. A dampered duct 43 provided with a damper 44 connects conduit 42 to air chamber 34. Below the auxiliary air heater 46 a duct 45 28 to flow back and forth over the Y heater heats some of the heated air from conduit 33 which is delivered via conduit 42 to the mill 2! for the purpose of drying the fuel. Since there is a positive pressure in the air at the top of.the main. air heater 25, air will flow through the heater tubes 4| and conduit 42 and the mill tov the fan IS without an additional fan. A damper 4'! is provided in conduit 42 for controlling the flow of superheated air to the mill. The temperature of the superheated air may be lowered by admitting some cooler air from chamber 34 at the front of the furnace or by admitting cold fresh air from cool air conduit 32 through duct 45. When no superheated air is required the damper 41 is closed and damper 46 in duct 45 opened so that fresh air flows upwardly through the auxiliary air heater 40 to Join-the heated air from main air heater 25 in conduit 33. When not required for superheating air the auxiliary 40 may thus be used for heating fresh cold air, thereby protecting the metal of the auxiliary heater from over-heating and at the same time maintaining the efficiency of the combined steam generating unit by using the auxiliary heater at all times.

What is claimed is: 1. In apparatus having a furnace from which the gases of combustion flow over an air preheater; an auxiliary air heater located in advance of said preheater in the direction of gas flow; a connection between the air outlet of said air preheater and one end of the air passage of said auxiliary air heater; an air outlet duct leading from the opposite end of the air passage of said auxiliary air heater to a point of use; another duct connecting the air inlet of said air preheater with said opposite end of the air passage of said auxiliary air preheater; a normally open and via said duct adjacent and substantially in said first-mentioned duct premitting heated air to flow from the outlet end or the main air preheater through said auxiliary air preheater to said point of use; and a normally closed damper in saidsecond mentioned duct shutting 01! communication between the inlet of said air preheater and said opposite end of said auxiliary air preheater, said normally closed damper acting when open to permit air to flow from the inlet end of said main air pre heater to said opposite end of the air passage of said auxiliary air preheater.

2. In a boiler, means forming a gas pass; steam generating tubes disposed in said gas pass: means forming another gas pass immediately parallel to said first pass and connected in series therewith; an air damper flow in one direction through the air preheater located in said second mentioned gas pass; means for supplying air to one end oi. the air passage of said air preheater and for withdrawing heated air from the opposite ends of said tubes; additional air heating tubes interspersed among the tubes of said bank adjacent and parallel with the tubes of said main air preheater and forming an auxiliary air preheater; means connecting said air preheater tubes at their outlet ends in series with said additional tubes at one end thereof; an pfltake duct connected to said additional tubes at their opposite ends for conveying heated air to-a point of use; another duct connecting the air inlet of said air preheater to the other ends -of said additional tubes; and dampers in each of said ducts acting when the damper in said first duct is open and that in said second duct is closed to cause the air to passage of said auxiliary air preheater and in series relation to said air preheater and acting when the positions of said dampers are reversed to cause the air to flow in the opposite direction through the air passage of said auxiliary air preheater and placing said air preheaters in parallel as to air flow.

GEORGE P. JACKSON. WARD S. PATTERSON. 

